by Jeffrey P. Broker, PhD
"It's my knee, Doc! I can't push the big gear or do the long ride anymore." These sad words from a disheartened bicycle racer should sound familiar to any clinician who regularly treats cyclists. Worse yet, cycling knee injuries seem to be on the rise. What is it about pedaling action that puts excessive stress on the knee joint? A closer look at knee injury patterns in cycling and potential pathomechanicals of pedaling is warranted.
The lower extremity in cycling is smoothly guided during each pedal stroke through a limited and well defined range of motion. Forces generated within the skeleton during pedaling also fluctuate in a steady, predictable, and nonviolent manner. Consequently, it is no wonder cyclists generally enjoy a freedom from such injuries as ligament sprains (caused by excessive joint motions), cartilage tears (resulting from excessive skeletal loading in conjunction with specific motions), and fatigue fractures (a repetitive impact phenomenon). Instead, the bane of the cyclist is more often the non-traumatic overuse injurythat affliction caused by repetitive and excessive loading of specific tissues.
The knee tops the list as the site where the greatest number of cycling-related overuse injuries occur. Higher intensity training methods, changes in bicycle/rider setup geometry (notably steeper seat tube angles), and the widespread use of clipless pedals are factors believed to be associated with an increase in the number and frequency of overuse knee injuries in recent years. A closer look at the knee injury picture indicates anterior knee injuries are most common among cyclists (patellar pain and patellar tendinitis leading the pack), followed by medial injuries (medial plica, retinacular thickening), and then lateral injuries (iliotibial band syndrome in particular).
Which cyclists sustain overuse knee injuries? Cyclists of every ability level are at risk. High-level competitive cyclists (category I and II, for example) train intensely to remain at their competitive best. A large percentage of knee injuries in this population result from training errors such as too many miles, too many hills, and use of too-large gears (or too-low a pedaling rate) early in the season.
Another causative factor among competitive cyclists suffering with knee pain are errors in bicycle adjustments. In my opinion, these adjustment errors, including maladjusted cleats and/or improperly placed saddles, generate misalignments in the lower extremity during the pedal stroke. Misalignments, although often subtle and difficult to see by visual inspection, can generate abnormally high cyclical loads in specific tissues. These loads become pathological at the intensity levels and training volumes at which these cyclists operate.
Touring cyclists, whose participation in the sport often involves occasional rides of significant length each year, often develop a knee overuse injury assignable to one specific ride. Patellar pain is the most frequent clinical finding in these cyclists, followed by iliotibial band syndrome. These sporadic high mileage riders sometimes can be classified as poor trainers, because they often do not train adequately for their touring rides. Bicycle maladjustments also are frequent in this group.
Recreational riders and individuals using bicycles and stationary trainers for aerobic exercise seek out orthopaedic treatment less frequently for their cycling-specific overuse knee injuries, so data concerning the nature and cause of these injuries are limited. It can be assumed, however, that knee injuries develop in these cyclists as mileage levels and/or training intensities rise, particularly if maladjustments in bicycle setup are present.
The cycling overuse knee injury phenomenon has not gone unnoticed by biomechanists. Early motion studies revealed that many cyclists reporting to clinics with knee problems exhibited nonlinear knee trajectories during pedaling. Imagine the trajectory the knee cap or tibial tuberosity follows in space as viewed from the front of the bicycle. In problematic cyclists, the trajectories of these knee landmarks often deviate significantly toward the top tube, or centerline of the bicycle during the downstroke. Some cyclists even brush their knees against the bicycle top tube during pedalling. As a result of these observations, a general relationship between internal rotation of the tibia (counterclockwise rotation of the right leg when viewed from above), knee valgus (a knocked kneed position), and knee injuries was proposed. Several clinicians and researchers began experimenting with orthoses and pedal cant in an attempt to elicit more linear knee motion in problematic cyclists. These efforts have been successful in managing knee pain in some cyclists.
In search of a more complete understanding of how some knee pain develops, biomechanists turned to the instrumented bicycle pedal for answers. One concept that surfaced was a potential relationship between knee pain and pedal torsion. The ramifications regarding bicycle setup and shoe/pedal system selection are potentially significant.
Torsional forces are present at the shoe/pedal interface during pedaling. The torsional force (or torque) arises as the foot in the shoe attempts to rotate or "twist" about an axis perpendicular to the pedal surface. Pedal torsion typically peaks midway through the power phase of the pedal cycle (downstroke), and is described to be internally directed. In other words, the foot attempts to rotate inward relative to the pedal (heel attempts to move out). During recovery (upstroke), torsion is externally directed, but lower in magnitude than the internally directed torsion seen during the power stroke.
The effect of this torque acting at the shoe/pedal interface on knee loading may be critical. The torque applied by the lower extremity to the pedal about an axis perpendicular to the pedal surface is, in a sense, transmitted to the knee joint. Since this torque at the shoe/pedal interface is known to be affected (and probably reduced) by the allowance for "float" within the pedal system, it is reasonable to suggest that the introduction of float into the pedal system may reduce the magnitude of the torque at the knee, and perhaps the frequency of overuse injuries.
In a study conducted at UCLA in the early 1990s, pedal interface type was found to affect the magnitude of pedal torsion. Specifically, floating clipless pedals reduced the power phase (downstroke) torsion relative to fixed clipless and toe-clip designs. The attenuation decreased both the internal and external peak torque.
Perhaps even more dramatic, experienced cyclists clinically diagnosed with overuse knee injuries in the UCLA study exhibited distinctly different pedal torsion patterns. Specifically, chronic anterior knee pain subjects demonstrated exaggerated internally directed peak torques during the power phase, increased rates of torsional load generation, and a longer duration of the applied internal torque. These results strongly suggest a link between pedal torsion and knee pain in cyclists. These results also offer support to the numerous subjective claims from cyclists that switching from fixed to floating pedals is an effective cure for pedaling knee pain.
Finally, revisiting the proposal that tibial rotation plays a role in knee pathology, a recent study conducted with U.S. National Team cyclists at the Olympic Training Center revealed that tibial rotation in elite cyclists appears to have a very weak correlation with torsion measured at the pedal. It appears that pronation and supination at the subtalar joint, which couple rotations of the tibia to motion within the foot, may be complicating our interpretation of lower limb action and pedal torsional loading. A detailed investigation of skeletal motion in the lower limb in conjunction with pedal force and torque measurements will be required to more fully understand this phenomenon.
The overuse knee injury problem in cycling will certainly not go away. Biomechanical research concerning the mechanics and pathomechanics of pedaling will continue to enhance our ability to treat and manage cyclists presenting with knee pain. It remains the challenge of the experienced clinician with a specialized knowledge of cycling to weigh the various factors including training history, musculoskeletal symmetry, alignment, functional status, rider/bicycle configuration, and pedaling technique in an attempt to keep riders riding longer, faster, and pain free.
Jeffrey P. Broker, PhD, is a senior sport biomechanist for the United States Olympic Committee in Colorado Springs and a consultant for Biomechanics Engineering in Colorado.